Non-aligned probes have been developed for atomic force microscopy (AFM), including strain gauge cantilever probes containing piezoelectric or piezoresistive material which produces a change in voltage or resistance in response to cantilever bending. Such probes were first disclosed in U.S. Pat. Nos. 5,229,606 and 5,266,801 and have since also described in U.S. Pat. Nos. 5,345,816, 5,345,815 and 5,321,977. Non-aligned STM probes and piezoresistive cantilever probes are used in the AutoProbe™ VP UHV AFM/STM sold by Park Scientific Instruments.
Atomic Force Microscopes are devices that provide three dimensional topographic images of surfaces. These devices are capable of providing resolution to atomic dimensions of surface features. In an Atomic Force Microscope (AFM), an extremely sharp tip is mounted on a small flexible lever. The tip is positioned on a surface such that the attractive forces between the surface and the tip and the repulsive force of the surface on the tip are very close to equilibrium so that the force of the tip on the surface is extremely low. If the tip is scanned laterally across a sample, the deformation of the tip will vary with the surface structure and this modulation versus lateral scan position can be used to produce an image of the surface. More typically, the sample may be servoed up and down such that the tip deformation (and thus the tip force) is kept constant during lateral scanning and the vertical adjustment signal versus lateral scan position produces a topographic map of the surface. A microscope of this type is described in U.S. Pat. No. 4,935,634, by Hansma et al. The deformation of the tip can be sensed in various ways, such as using the tunneling effect off the backside of the tip as described in a patent by Binnig, optical means such as beam deformation as described in Hansma, or interferometry. Typically, most AFMs mount the tip on a low spring-constant cantilever and sense deformation by monitoring the change in angle of reflected light off the backside of the cantilever. AFMs can operate directly on insulators as well as conductors and, therefore, can be used on materials not directly accessible to other ultra-high resolution devices such as Scanning Electron Microscopes (SEMs) or Scanning Tunneling Microscopes (STMs).
The tip in an AFM must be positioned with extreme accuracy in three dimensions relative to a sample. Motion perpendicular to the sample (z-axis) provides surface profile data. Motion parallel to the surface generates the scanning. In a typical system, the image is developed from a raster type scan, with a series of data points collected by scanning the tip along a line (x-axis), and displacing the tip perpendicularly in the image plane (y-axis); and, repeating the step and scan process until the image is complete. The precise positioning in x, y, and z is usually accomplished with a piezoelectric device. Piezoelectric devices can be made to expand or contract by applying voltages to electrodes that are placed on the piezoelectric material. The motions produced by these piezoelectric scanners can be extremely small, with some scanners having sensitivities as low as tens of angstroms per volt. The total deformation possible for these scanners is typically less than 200 microns. Scanners with different sensitivities are used for different applications, with low sensitivities used for atomic resolution images, and higher sensitivity scanners used for lower resolution, larger area images. The design of the piezoelectric scanners, including the shape of the scanner and the placement of electrodes, is well known in the art.
In an AFM, either the sample can be attached to the scanner and the tip held stationary or the tip can be attached to the scanner and the sample fixed. Typically, most existing AFMs scan the sample. This invention will describe, and the drawings will represent the case where the sample is scanned; but, the invention applies equally well to either case.
As the sample is scanned in x and y, the z axis movement is closely coupled to the tip deformation. In an AFM, either the tip deformation can be monitored as the sample is scanned, or the Z position can be varied to maintain the deformation constant with feedback. This constant deformation is called the setpoint and can be set by the control system. Modulating the z position with feedback is useful for controlling and minimizing the contact force between tip and sample, and also allows the AFM to be used for other measurements, such as stiffness.
Scanning Probe Microscopes (SPMs) scan sharp probes over a sample surface and make local measurements of the properties of a sample surface. One common example is the atomic force microscope, also known as the scanning force microscope, that scans a sharp stylus attached to a flexible spring lever (commonly called a cantilever) over a sample surface. By measuring motion, position or angle of the free end of the cantilever, many properties of a surface may be determined including surface topography, local adhesion, friction, elasticity, the presence of magnetic or electric fields, etc. Other SPMs include the scanning tunneling microscope, the scanning near-field optical microscope, the scanning capacitance microscope, and several others.
One example of a delicate probe is the cantilever chip used in atomic force microscopy. The typical cantilever is 10-500 micrometers long, 10-50 micrometers wide and 0.5-5 micrometers thick, and the cantilever is often fabricated on a support substrate that is roughly 1.5 mm long, 3 mm wide, 0.5 mm high. Cantilevers are also formed out of single wires or thin metal beams, etc. Because of the delicate nature of the probes and the alignment described below, replacement of the probe in some SPM designs may take many minutes, as described in U.S. Pat. No. 5,376,790, assigned to Park Scientific Instruments. During this time, the SPM instrument is usually unavailable for use, so minimal probe exchange and alignment time is essential for high sample throughput. As scanning probe microscopes become more and more widely used, there is increasing pressure to develop instruments that can be operated more quickly and used by less-skilled operators, or even driven automatically without operator intervention.
In an attempt to solve this problem U.S. Pat. No. 5,760,675 issued Jun. 2, 1996 provides an ultra-thin Mo—C film with a thickness as small as 0.15 nanometers that still maintains an electrical continuity. The film can be prepared on insulating or semiconducting substrates of flat surface by the sputtering method using a Mo—C target directly, or by a reactive-sputtering method with a Mo target in an ambient Ar/C2H2 mixture gas at a wide range of substrate temperature.
The electrically continuous and ultra-thin Mo—C film is sensitive to a mechanical force per unit area because it is ultra-thin and there are few atoms sustaining the force. When the Mo—C film has a thickness of only a few atomic distance, for example, about 1 nanometer, the extremely small mechanical stress/strain can change the atomic distances in the film enough to affect the electrical conductivity of the film, because the distance between atoms is the crucial factor of the electrical conductivity for a given material. Therefore, this ultrathin Mo—C film can serve as an excellent piezoresistive material.
Another aspect of the Mo—C film which is crucial to such a piezo-device is that the ultra-thin Mo—C can be easily prepared on substrates with any flat surfaces of insulator or semiconductor. The film's quality of piezoresistivity is very weakly dependent on the stoichiometry of Mo0.5+xC0.5−x within the moderate range up to x=0.25 and the deposition temperature as well as the substrate. These properties are essential to the fabrication of such integrated circuits.
U.S. Pat. No. 5,266,801 makes use of the piezoelectric or piezoresistive materials to measure the strain on the cantilever, but it still has the noise problem left irresolvable for the signal along the connecting wire. Also, the exterior transfer and processing circuit occupies a relatively large space and complicates the system structure.
U.S. Pat. No. 5,400,647 measures, by using an Atomic Force Microscope, the transverse force which is related to the magnitude of the frictional force. The Atomic Force Microscope makes use of an optical way of measuring the deformation of the cantilever. Similar to other prior art Atomic Force Microscopes, since they have many optical elements, their system spaces are relatively large, their structures are relatively complicated, and the noise problems of their connecting wires still exist.
U.S. Pat. No. 5,468,959 is a method for measuring the surface not the particular elements of the probe apparatus itself. Although the patent does mention the probe, however, this probe is not the focus of the patent. This patent mainly describes the use of capacitor and electro-static force, and the measurement of displacements and external electro-static force.
Another suitable MEMS detector for use in this invention comprises a resonating element wherein the resonate frequency of the resonator is a sensitive function of it's mass. Two primary types of resonators have been utilized previously, microcantilevers and miniature tuning forks. U.S. Pat. No. 5,719,324 (Thundat 1998) discloses micorcantilevers modified with selective binding agents and methods for measuring the resonate frequency or deflection. U.S. Pat. No. 6,289,717 (Thundat 2001) describes a sensor apparatus using a microcantilevered spring element having a coating of a detector molecule such as an antibody or antigen. A sample containing a target molecule or substrate is provided to the coating. The spring element bends in response to the stress induced by the binding which occurs between the detector and target molecules. Deflections of the cantilever are detected by a variety of detection techniques. The microcantilever may be approximately 1 to 200 micrometers long, approximately 1 to 50 micrometers wide, and approximately 0.3 to 3.0 micrometers thick. A sensitivity for detection of deflections is in the range of 0.01 nanometers.
U.S. Pat. No. 6,528,026 whish describes devices and methods for measuring the properties of combinatorial arrays of polymers using resonators which may include miniature tuning forks. This system is particularly useful for measuring intrinsic polymer properties such as Tg and molecular weight. Other publications cited in the '026 patent include “The Oscillation Frequency of a Quartz Resonator in Contact with a Liquid,” K Keiji Kanazawa and Joseph G. Gordon II, Analytica Chimica Acta, vol. 175, pp. 99-105, 1985. 1 MHz quartz length extension resonator as a probe for scanning near-field acousitic microscopy, Thin Solid Films, A. Michels, F. Meinen, T. Murdfield, W. Gohde, U. C. Fischer, E. Beckmann and H. Fuchs, vol. 264, pp. 172-195, 1995. Piezoelectric quartz resonators or mechanical oscillators can be used to evaluate the viscosity of reaction mixtures, as well as a host of other material properties, including molecular weight, specific gravity, elasticity, dielectric constant, and conductivity. In a typical application, the mechanical oscillator, which can be as small as a few mm in length, is immersed in the reaction mixture. The response of the oscillator to an excitation signal is obtained for a range of input signal frequencies, and depends on the composition and properties of the reaction mixture. By calibrating the resonator with a set of well characterized liquid standards, the properties of the reaction mixture can be determined from the response of the mechanical oscillator. Further details on the use of piezoelectric quartz oscillators to measure material properties are described in co-pending U.S. patent application Ser. No. 09/133,171 “Method and Apparatus for Characterizing Materials by Using a Mechanical Resonator,” filed Aug. 12, 1998, which is herein incorporated by reference.
Although many different kinds of mechanical oscillators currently exist, some are less useful for measuring properties of liquid solutions. For example, ultrasonic transducers or oscillators cannot be used in all liquids due to diffraction effects and steady acoustic (compressive) waves generated within the reactor vessel. These effects usually occur when the size of the oscillator and the vessel are not much greater than the characteristic wavelength of the acoustic waves. Thus, for reactor vessel diameters on the order of a few centimeters, the frequency of the mechanical oscillator should be above 1 MHz. Unfortunately, complex liquids and mixtures, including polymer solutions, often behave like elastic gels at these high frequencies, which results in inaccurate resonator response.
Often, shear-mode transducers as well as various surface-wave transducers can be used to avoid some of the problems associated with typical ultrasonic transducers. Because of the manner in which they vibrate, shear mode transducers generate viscous shear waves instead of acoustic waves. Since viscous shear waves decay exponentially with distance from the sensor surface, such sensors tend to be insensitive to the geometry of the measurement volume, thus eliminating most diffraction and reflection problems. Unfortunately, the operating frequency of these sensors is also high, which, as mentioned above, restricts their use to simple fluids. Moreover, at high vibration frequencies, most of the interaction between the sensor and the fluid is confined to a thin layer of liquid near the sensor surface. Any modification of the sensor surface through adsorption of solution components will often result in dramatic changes in the resonator response.
U.S. Pat. No. 6,553,318 describes methods to measure various physical properties of polymers using resonators cites therein “Visco-elastic Properties of Thin Films Probed with a Quartz Crystal Resonator,” D. Johannsmann, F. Embs, C. G. Willson, G. Wegner, and W. Knoll, Makromol. Chem., Macromol. Symp., vol. 46, 1991, pp. 247-251
Recently researchers at Arizona State University have referred to a miniature tuning fork based sensor using polymer “wires”.
Their work refers to the use of a chemical vapor sensor by using a quartz tuning fork array. The sensor array is based on the detection of mechanical response of thin polymer wire stretched across the two prongs of quartz tuning forks (2 mm×1 mm×0.2 mm). When the fork is set to oscillate, the wire is stretched and compressed by the two prongs, showing a resonance spectrum of unique features. Upon exposure to the analyte, the polymer mechanical properties are changed and a frequency and/or amplitude shift takes place in an extension proportional to the analyte concentration. Every fork of the array is modified with a particular polymer wire and oscillates with its proper frequency. So, a multiple-peak spectrum is obtained when a linear frequency sweep is applied. After chemical vapor injection, some of the polymer wires suffer chemical absorption and the resulting spectrum changes allow us to discriminate the nature of the chemical vapor. Their work shows testing of five different polymer materials to sense different polar and non-polar vapors. The responses towards ethanol vapor injections with various concentrations have been demonstrated and very low detection limits were achieved.